In semiconductor processing, a covering layer is normally applied over the surface of the semiconductor wafer as a final process step to protect circuits thereon. The protective layer typically comprises approximately 15000 Angstroms of nitride formed by chemical vapor deposition of SiH.sub.4 and ammonia (NH.sub.3). The wafer may then be shipped to other locations for further processing, and the nitride will provide protection against shipping damage. Additionally, the nitride will protect the circuits from dust and other contaminants during the sawing of the wafer into individual chips.
Unfortunately, nitride (frequently called compressive nitride) induces a compressive stress across the surface of the wafer which tends to cause the normally flat surface to bow. If the stress is high enough, microcracks may form in the chips which may destroy the circuits thereon. Since the microcracks are generally too small to be detected by observation, some test must be conducted to check for the possibility of damage to the wafer.
One test device commonly used is a stress gauge such as is available from Ionic Systems of San Jose, Calif. The gauge must be calibrated by using a wafer having a known stress value (such as a wafer obtained from the National Bureau of Standards) measured in 10.sup.-9 dynes/ cm.sup.2. The gauge uses a light source and fiber optics. The initial light beam strikes the backside of the wafer being measured and is reflected back to a fiber optic beam splitter. A fiber optics receiver sends the light to a fiber optic detector which measures the intensity of the returning light. A digital readout displays a number which represents a known wafer stress. A wafer to be tested is then placed on the gauge and a numerical readout is obtained which must be converted to a stress value by comparison to the known wafer readout.
The stress gauge is susceptible to external factors that tend to reduce the accuracy of the readout. Since the test readings are based on a supposedly known value as a standard, any error in the standard will cause error in the test values. Stress tends to diminish over time so that the known value (which is questionable in the first place) may change. Additionally, temperature and humidity affect the gauge as well as the wafer, and therefore a new base reading must be taken any time the temperature or humidity changes. Finally, the gauge requires a 24 to 48 hour warm-up to obtain the best results.
Another testing device such as the Lang X-ray Topography Camera uses X-rays to scan across a wafer to determine a radius of curvature. Prior to testing, the Lang device must be calibrated using an uncoated wafer that is assumed to be perfectly flat. To conduct a test on a coated wafer, X-rays penetrate the protective coating and are diffracted by the crystalline structure of the silicon. The diffraction of the X-rays produces deflected rays of light that may be detected and evaluated to determine the radius of curvature of the wafer. Once the radius of curvature is known, the stress on the surface may be calculated.
The Lang device is relatively expensive and requires highly skilled and trained personnel to operate, maintain and calibrate. Thus, there is a need for a relatively low cost device and method that is easy to operate and maintain and does not require the use of a supposedly standard wafer to calibrate.